Our probes and landers still look for life "as we know it." But we're already aware that even terrestrial life goes beyond what we once considered possible. We should use that experience, or we may literally step on alien life.

Extraterrestrial life is a staple of SF and the focus of astrobiology and SETI. Yet whereas SF has populated countless worlds with varying success, from Tiptree's haunting Flenni (Your Haploid Heart) to Lucas' annoying Ewoks, real ETs remain stubbornly elusive: nobody has received a transmission demanding more Chuck Berry, and the data from the planetary probes are maddeningly inconclusive. Equally controversial are the shadowy forms on Martian asteroid ALH84001, although the pendulum has swung toward cautious favoring of the biological possibility after scientists discovered nanobacteria on earth and water on Mars.

In part, we're hobbled by the limits of our technology, including the problems of sample contamination and method-specific artifacts. But we're also severely limited by having a single life sample. Despite its dizzying variations in form and function, extant terrestrial life arose from one source. We know this because our genetic blueprint and its associated molecular machinery are identical across the three domains (archaea, eubacteria, eukarya). So to be able to determine if something is alive, we need to decide what is universal and what is parochial. We stumble through redefinitions each time our paradigms shift or our techniques achieve higher resolution. Worse yet, our practices lag considerably behind our theories.

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If life elsewhere is similar enough to us or sufficiently advanced, there will be no way to mistake it for anything else: when something tries to shake your hand, when someone beams a message that is recognizably mathematics or music, you do not need further tests. There is an area at the boundary of the exotic, though, which contains both the very different and the relatively primitive. For these, more exhaustive criteria will be needed to distinguish life from non-life.

This would be especially true of lifeforms that aren't based on carbon or don't use water as solvent, whose biochemistry would be nothing like ours. For these, we would have to fall back to the highest-order definition of life: an open system with negative entropy, emergent properties and ability to adapt and evolve, with an inner code which ensures that there will be strong continuity of form and function as the organism reproduces. This leaves room for Forward's neutron-star cheela (Dragon's Egg), Lem's sentient ocean (Solaris) and the silicon-based Horta in Devil in the Dark.

A few years ago, Dr. Dirk Schulze-Makuch of Washington State University suggested that the Viking probes may have inadvertently destroyed Martian bacteria. He theorized that if their optima differ significantly from "median" terrestrial bacteria, the tests of the probes – heating, adding water – would be lethal. His speculations, if correct, could explain and reconcile the contradictory results from the biological experiments conducted by the Viking landers.

This shows how our lack of an independent life sample limits our horizons. In 1976 and 1977, the years of the Viking landings, extremophilic bacteria were unknown. Even after their discovery, it took heroic measures to propagate them once they were removed from their native habitats. Also unknown were the thriving communities of fragile, gelatinous animals living in the ocean depths: the methods used to capture samples shredded them to confetti. Something similar may happen when we look for life under Europa's ice sheet or the Mars polar cap; in the geysers of Enceladus, Titan's hydrocarbon lakes, the clouds of Venus and Jupiter; or on the tidally locked earth-like planets of dim, flare-racked Gliese 581. For example, the heat of the drill alone may exterminate Europan organisms.

Even if we find technologically advanced sophonts (a lovely coinage by Anderson, derived from the Hellenic word for wisdom), it's far from certain that we will be able to communicate with them. But I think that those who are still starry-eyed about extraterrestrial life find the quest compelling for reasons beyond its potential seismic impact on biology and culture: it embodies the desire of humanity for companions amid the sea of stars, a potent myth and an equally potent engine for exploration.